In the application of two part acrylic adhesives, the monomeric component, i.e. comprising the polymerisable acrylic monomer, the catalyst, and the optional ingredients, e.g. stabilizers, antioxidants, polymerisation inhibitors and toughening agents such as chlorosulphonated polyethylene or low molecular weight reactive elastomers such as a vinyl-terminated polybutadiene/acrylonitrile, and the initiator component, usually comprising an amine/aldehyde condensate, are usually applied separately each to a different one of the two surfaces to be joined, the two surfaces then being brought together and clamped whilst the adhesive cures, the cure being initiated by contact between the superimposed layers or films of primer, i.e. the initiator, and the monomeric component. In an alternative technique, the primer and the monomeric component can be applied both to the same surface, or to both surfaces, before those surfaces are brought into contact. Whilst such techniques are satisfactory when applying the adhesive between closely mating surfaces, i.e. with gap widths between the mating surfaces of the order of 0.05 mm or less, such techniques are not so successful in the sealing or bonding of larger gaps, i.e. greater than about 0.25 mm, due to a non-uniformity of cure, or poor "cure through". In this connection, in certain industries, for example, the vehicle manufacturing industry, gap sizes of 3 to 4 mm are commonplace and cannot presently be sealed, at least not satisfactorily, with conventional acrylic adhesives.
To overcome the problem of poor cure through on larger gap sizes, the acrylic monomer component and the initiator component can be pre-mixed immediately prior to application or injection of the adhesive, and for this purpose dual adhesive applicators and injection devices, e.g. self-proportioning, twin pack cartridge dispensers, have been developed, not exclusively in the field of acrylic adhesives and sealants, but in other adhesive fields as well, for the simultaneous ejection and mixing of the two components in predetermined proportions.
However, such self-proportioning, twin pack cartridge dispensers present a further problem in that, for accurate proportioning of the two components the relative proportions of the two components, on a volume basis, must not be greater than about 10:1. Traditional two part acrylic adhesive compositions are, however, used at much higher ratios, for example, at volume ratios of the order of 50:1 or more, i.e. the volume ratio of the acrylic monomer component to the initiator component.